Baldet, P. | bioRxiv Science

bioRxiv2022

Blue light promotes ascorbate synthesis by deactivating the PAS/LOV photoreceptor that inhibits GDP-L-galactose phosphorylase

Ascorbate (vitamin C) is one of the most essential antioxidants in fresh fruits and vegetables. To get insights into the regulation of ascorbate metabolism in plants, a mutant producing ascorbate-enriched fruits was studied. The causal mutation, identified by a mapping-by-sequencing strategy, corresponded to a knock-out recessive mutation in a new class of photoreceptor named PAS/LOV protein (PLP, Solyc05g07020), which acts as a negative regulator of ascorbate biosynthesis in tomato. This trait was confirmed by CRISPR/Cas9 gene editing, and further found in all plant organs, including fruit that accumulated 2-3 times more ascorbate than in the WT. The functional characterization revealed that PLP interacted with the two isoforms of GDP-L-galactose phosphorylase (GGP), known as the controlling step of the L-galactose pathway of ascorbate synthesis. The interaction with GGP occurred in the cytoplasm and the nucleus, but was abolished when PLP was mutated. These results were confirmed by an optogenetic approach using an animal cell system, which additionally demonstrated that blue light modulated the PLP-GGP interaction. Assays performed in vitro with heterologously expressed GGP and PLP showed that PLP is a non-competitive inhibitor of GGP that is inactivated after blue light exposure. This discovery sheds light on the light-dependent regulation of ascorbate metabolism in plants.

plant biology↗

bioRxiv2019

Mitochondrial ascorbate synthesis acts as a pro-oxidant pathway and down-regulate energy supply in plants

Attempts to improve the ascorbate (AsA) content of plants are still dealing with the limited understanding of why exists a wide variability of this powerful anti-oxidant molecule in different plant sources, species and environmental situations. In plant mitochondria, the last step of AsA synthesis is catalyzed by the enzyme L-galactone-1,4-lactone dehydrogenase (L-GalLDH). By using GalLDH-RNAi silencing plant lines, biochemical and proteomic approaches, we here discovered that, in addition to accumulate this antioxidant, mitochondria synthesize AsA to down-regulate the respiratory activity and the cellular energy provision. The work reveals that the AsA synthesis pathway within mitochondria is a branched electron transfer process that channels electrons towards the alternative oxidase, interfering with conventional electron transport. It was unexpectedly found that significant hydrogen peroxide is generated during AsA synthesis, which affects the AsA level. The induced AsA synthesis shows proteomic alterations of mitochondrial and extra-mitochondrial proteins related to oxidative and energetic metabolism. The most identified proteins were known components of plant responses to high light acclimation, programmed cell death, oxidative stress, senescence, cell expansion, iron and phosphorus starvation, different abiotic stress/pathogen attack responses and others. We propose that changing the electron flux associated with AsA synthesis might be part of a new mechanism by which the L-GalLDH enzyme would adapt plant mitochondria to fluctuating energy demands and redox status occurring under different physiological contexts.

plant biology↗